A microwave sounds like the least off-grid appliance imaginable, and for a tent it is. For a van with a decent battery it is more defensible than it looks, because the thing that matters is not how many watts it draws but how many watt-hours a job costs. A microwave draws a lot for a very short time. Reheating a bowl of stew is a three-minute job.
The number on the front is not the number that matters. A microwave sold as 800 W is quoting its cooking output, measured by heating a litre of water under IEC 60705. What it draws from the supply is higher, because the magnetron and its electronics are not perfectly efficient. The figure you need for battery planning is the input power on the rating plate, usually inside the door frame or on the back, and it is the one nobody quotes in the listing.
So buy on the rating plate, not the badge. A “700 W” model with a low input figure is a genuinely useful thing to have; an 1,100 W one is a different proposition entirely.
What it is good for here is reheating and rehydrating rather than cooking: porridge, a bowl of something made last night, couscous. What it is bad at is everything this site actually does, which is building flavour in a pan. It earns its place as a second method, not a first.
If you are reheating rice, the rules do not change because the heat source did: rice, reheating and cooking without a fridge.